The present invention relates to a mixer in an injection molding machine. More particularly, the present invention relates to a mixer apparatus and method to improve the homogeneity of molten material in an injection molding machine and hot runners.
The large number of variables in the injection molding process creates serious challenges to creating a uniform and high quality part. These variables are significantly compounded within multi-cavity molds. Here we have the problem of not only shot to shot variations but also variations existing between individual cavities within a given shot. Shear induced flow imbalances occur in all multi-cavity molds that use the industry standard multiple cavity xe2x80x9cnaturally balancedxe2x80x9d runner system whereby the shear and thermal history within each mold is thought to be kept equal regardless of which hot-runner path is taken by the molten material as it flows to the mold cavities. These flow imbalances have been found to be significant and may be the largest contributor to product variation in multi-cavity molds.
Despite the geometrical balance, in what has traditionally been referred to as xe2x80x9cnaturally balancedxe2x80x9d runner systems, it has been found that these runner systems can induce a significant variation in the melt conditions delivered to the various cavities within a multi-cavity mold. These variations can include melt temperature, pressure, and material properties. Within a multi-cavity mold, this will result in variations in the size, shape and mechanical properties of the product. Though the effect is most recognized in molds with eight or more cavities, it can create cavity to cavity variations in molds with as few as two cavities.
The flow imbalance in a mold with a geometrically balanced runner is created as a result of shear and thermal variations developed across the melt as it flows through the runner. The melt in the outer region (perimeter) of the runner""s cross-section experiences different shear and temperature conditions than the melt in the center region. As flow is laminar during injection molding, the position of these variations across the melt stream is maintained along the length of the runner branch. When the runner branch is split, the center to perimeter variation becomes a side to side variation after the split. This side to side variation will result in variations in melt conditions from one side to the other of the part molded from the runner branch. If the runner branches were to split even further, as in a mold with 4 or more cavities, there will exist a different melt in each of the runner branches. This will result in variations in the product created in each mold cavity. It is important to note that as consecutive turns and/or splits of the melt channel occur, the difference in melt temperature and shear history is further amplified. This cumulative effect is clearly recognized in large multi-cavity molds where the runner branches split and turn many times.
In an attempt to reduce this variation, the prior art has been directed at various mixing devices that are located within the runner nozzle which is typically just prior the mold cavity.
U.S. Pat. No. 5,405,258 to Babin shows a hot runner nozzle having a torpedo which is used to conduct heat absorbed from the upstream melt along its length to the gate area. The torpedo is positioned within the melt stream and supported by spiral blades that induce a swirling motion to the melt as it flows past them.
U.S. Pat. No. 5,849,343 to Gellert et al. shows a valve gated nozzle having a stem guiding nozzle tip that causes the melt to divide from a cylindrical flow to annular flow as it flows by the valve stem.
U.S. Pat. No. 4,965,028 to Manus et al., U.S. Pat. No. 5,513,976 to McGrevy, European Patent 0 546 554 to Gellert, and German Patent DE 32 01 710 to Gellert all teach various ways to mix the melt in a hot runner nozzle.
U.S. Pat. No. 5,545,028 to Hume et al. shows a hot runner tip having a semi-torpedo style in which the outer surface of the torpedo includes a flow channel that converts a single cylindrical inlet flow to an annular flow passing by the tip.
In spiral mandrel dies used in extrusion molding, single or multiple incoming cylindrical melt streams can be converted to a single annular outflowing stream in a continuous process like blown film extrusion molding. U.S. Pat. Nos. 5,783,234 and 5,900,200 to Teng show one application of this in a hot runner valve gated nozzle in which the spiral elements are formed in a comparatively large diameter valve stem and positioned relatively distant from the mold cavity.
U.S. Pat. No. 5,683,731 to Deardurff et al. shows a melt flow redistributor. This device is an annular plug that is inserted at the intersection of branching hot runner channels. A first diverter is included for distributing the outside boundary later of the melt into a plurality of hot runner branches. A second diverter is included that distributes the center boundary layer of the melt into a plurality of hot runner branches for mixture with the outside boundary layer. In operation, this device acts more as a flow flipper than a mixer, with very little mixing and melt homogenizing occurring.
None of the prior art teaches an apparatus for reducing the variation within a melt flow as it travels through the runner branches by gradually changing the flow from all helical to all annular. The prior art attempts to reduce the variation within the melt by altering the flow of the melt within the nozzle. By the time the melt reaches the nozzle, there exists a large variation in the melt due to the cumulative effects of the flow imbalance. Indeed, the efficiency of the prior art will benefit from the use of the present invention because the melt that reaches the mixers of the prior art that are located at the nozzle will have less variations in thermal and shear properties, thereby reducing the amount of mixing required by the nozzle mixing device and thereby improving overall part quality.
There exists a need, therefore, for an apparatus and method for use in injection molding machines that will reduce the cumulative effects of flow imbalance as it splits into multiple branches within the runner system, thereby reducing the variations that occur in the finished product of a multi-cavity system.
The primary objective of this invention is to provide a method and apparatus for reducing the flow imbalances that occur in an injection molding machine and runner system thereby creating high quality plastic articles.
Another objective of this invention is to increase the efficiency of melt mixers that are installed in the nozzle of an injection molding machine by reducing flow imbalance effects.
Still another objective of the present invention is to provide a method and apparatus for improving the homogeneity of the melt and reduce the effects of flow imbalance within a multi-cavity mold.
The foregoing objects are achieved by the installation of the present invention in an injection molding machine, particularly in the runner system of a multi-cavity mold. The present invention includes a flow channel for resin flow having an inlet area for receiving molten resin, an outlet area for transferring molten resin further downstream; an elongated shaft extending in the flow channel, such as a guide or torpedo, adjacent the outlet area; at least one spiral groove formed in the inner surface of the flow channel and facing the shaft that decreases in depth towards the outlet area, with lands adjacent said groove that increases in clearance towards the outlet area, wherein a helical flow path of resin is provided through the spiral groove and an axial flow path of resin is provided over the lands. Preferably, a sleeve is provided in the flow channel adjacent the elongated shaft, wherein the groove is formed in the sleeve. A portion of the lands are generally bonded, press-fit or taper locked to the shaft and the lands increase in clearance with respect to the shaft towards the outlet area.
The injection molding method of the present invention includes; supplying molten resin to a flow channel having an inner surface thereof in an injection molding machine runner, which flow channel extends in said runner from an inlet area to an outlet area for transferring molten resin further downstream in an injection molding machine; providing an elongated shaft in the flow channel adjacent the outlet area; transferring the molten resin to at least one spiral groove, said groove formed in the inner surface of the flow channel, and transferring the resin from the groove to the outlet area; decreasing the depth of the groove towards the outlet area and increasing the clearance of the lands toward the outlet area; thereby flowing the resin in a helical flow path through the spiral groove and in an axial flow over the lands.
Further features of the present invention will appear hereinbelow.